This invention relates to pattern tracers and, more particularly, to pattern tracers of the type which have means for varying the forward offset of the pattern sensing element for different tracer speeds and means for selectively varying the lateral offset of a tracing element with respect to the pattern for different kerfs, or cutting thicknesses associated with different cutting tools.
Pattern tracers are well known, and reference may be made to the numerous patents issued in this field for background information. Examples of such patents include U.S. Pat. Nos. 2,499,178 of Berry et al; 2,868,993 of Henry; 2,933,668 of Brouwer; 3,017,552 of Brouwer; 3,209,152 of Brouwer; 3,366,857 of Jewell et al.; 3,386,786 of Kaisler et al.; 3,479,513 of Simpson et al.; 3,534,162 of Hannappel et al.; 3,544,221 of Putnam; 3,624,574 of Montagu; 3,704,372 of Parker et al.; 3,704,373 of Bardwell et al.; 3,711,717 of Rich et al.; 3,717,332 of Luker et al.; 3,725,761 of Webber; 3,727,120 of Jewell et al.; 3,767,923 of Bardwell; 3,812,412 of Hahn et al.; 3,860,862 of Dell et al.; 3,883,735 of Murphy et al.; 3,902,783 of Bodlaj; 3,920,316 of Daguillon; 3,932,743 of Sitnichenko et al.; 3,946,166 of Wossidlo; 3,959,673 of Montagu; 3,995,154 of Schlick et al.; 4,002,900 of Sitnichenko et al.; 4,011,003 of Dragt; 4,021,096 of Dragt; 4,039,246 of Voigt; 4,049,962 of Kallen; 4,063,287 of van Rosmalen; 4,073,567 of Lakerveld et al.; 4,081,671 of Bohme et al.; 4,090,112 of Selverstone; 4,100,576 of Ditthardt; 4,123,146 of Dragt; 4,128,297 of Bourne; 4,128,481 of Schreyer; 4,129,814 of Francke; and 4,135,119 of Brosens.
Briefly, such tracers are provided with an optical sensor for photometrically detecting the pattern being traced and means for controlling the movement of a tracing element to follow the pattern in accordance with pattern detection signals generated by the sensor. Typically, the movement of a tool, such as a cutting torch, is controlled in accordance with the movement of the tracing element. Due to different kerfs, i.e. different thickness dimensions of the cut made by the cutting tool, such tracers are designed to follow the pattern at selected lateral offsets from the pattern, equal to half of the kerf of the tool being controlled.
One of the problems encountered in tracing a pattern is due to momentum. This problem is encountered whenever it is attempted to change the direction of the tracing element to follow a sharp, or small radius, turn in the pattern. These are the same forces which require a driver of an automobile to proceed slower around a sharp turn than when on a straight-away or gradual turn. Unfortunately, in many applications the pattern must be traced at a constant speed over the entire pattern path length including any sharp turns in the pattern. Theoretically, in order for the velocity to remain constant, the acceleration must approach infinity as the pattern turn becomes sharper, i.e., as the turn radius approaches zero.
Accordingly, in a real system where the force available to provide acceleration is limited, a certain degree of deviation from a given pattern path must be permitted. If a driver of an automobile enters a sharp turn in the road at a speed in excess of that at which the frictional forces of the tires can overcome the momentum of the automobile forcing it to go straight, then the automobile will miss the turn, and control of the automobile will be lost. Likewise, a tracing element will miss a turn in the pattern, if for a given velocity and a given pattern turn radius, the momentum cannot be overcome by the driving force controlling the movemennt of the tracer.
The phenomenon is illustrated in FIG. 1 of the accompanying drawings, in which a tracing element is being controlled to trace a pattern, such as a line pattern, from a point A to a pattern turn at point C and then to another point D. Because of the sharpness of the turn at point C, momentum will cause the tracing element to overshoot the turn and deviate from the pattern, as shown by the broken-line tracing path adjacent to point C. If the overshoot is extensive, the tracing element may be caused to continue off the pattern rather than ultimately return, as illustrated.
It is necessary for a driver to anticipate a sharp turn and to begin "cutting" the corner on the inside of the turn if his speed is too high in order to stay on the road and avoid missing the turn. Likewise, in pattern tracers, it is necessary to anticipate a sharp turn in the pattern and deviate from the pattern path. In known pattern tracers this anticipation has been achieved through use of a sensor which is offset forward of the tracing element, or reference point 0, to sense a change in pattern direction, or pattern turn, before that turn is reached by the tracing element. This forward offset is illustrated in FIG. 2 of the accompanying drawings in which the tracer is provided with a sensor that detects the pattern line ACD around the perimeter of an offset circle having an actual offset radius R from the tracing element O.
Once the pattern turn is detected, then the appropriate control, or drive, forces are applied to the tracing element to cause it to begin turning before the pattern turn is actually reached by the tracing element. The tracing element thus follows a path, shown in broken line (FIG. 3), deviating from the pattern path and "cutting the corner" or "rounding" the turn at point C. The turn of the tracing element begins when it is located at a distance from the turn at point C equal to the forward offset radius distance R which is when the pattern turn is first detected.
The amount of offset that is required to achieve the closest tracing of the pattern while avoiding overshoot is a function of both the speed of the tracing element and the maximum force available in the tracing element driving system. As the driving system becomes weaker, the the degree of offset required must become greater. Likewise, for a given driving system, if the tracer speed is increased, then the degree of offset required must also increase.
As the offset becomes greater, the deviation of the tracer path from the pattern becomes larger, and the turns of the tracer path become more rounded. Thus, it is desirable to avoid an offset greater than that which is needed to avoid overshoot.
In commercial practice, the balance of cost, driver power and permitted deviation is determined by the user and supplier of the tracing equipment. However, regardless of this balance, the forward offset required for the maximum speed of the tracing element is, of course, greater than that needed for speeds of the tracing element which are less than the maximum speed. A forward offset greater than needed will result in a greater deviation than necessary and a poorer quality tracing than could otherwise be obtained.
Consequently, a need exists to vary the offset for different tracer speeds. Known techniques for varying the forward offset as a function of tracer speed have been based on physical alteration of a sensor sytem. One tracer system is known which requires changing the magnification lenses for the sensor, so that it perceives a greater or lesser portion of the pattern in front of the tracing element. Another known tracing system requires changing the position of the sensor relative to the tracing element. In yet another system, a rotating mirror is used to direct the light from the pattern to the sensor, and the angle of the mirror is altered in response to centrifugal or electrical forces related to tracer speed to change the offset. These techniques unfortunately require a mechanical alteration of the sensing system each time a different speed is selected. All except the last technique discussed above, also unfortunately require the operator to physically interchange or move different parts of the sensing system for different speeds.
One other system is known which automatically varies the actual forward offset of a mechanically rotatable sensor with changes in tracer speed. In this system, a plurality of different sensor elements are provided in a sensor head which periodically, mechanically scans the pattern. The different sensing elements are associated with different forward offsets and are selected in accordance with the tracer speed. The necessary rotary scanning control for the sensor, however, significantly adds to the cost of this system.
Another problem with known tracers is that adjustment to the actual forward offset requires change to the kerf or lateral offset adjustment in order to maintain the same lateral offset kerf. This is due to the fact that in these sensors the lateral offset for a given kerf setting is partially based on the actual forward offset of the pattern sensor.